Breast cancer biology seems to indicate that breast cancer metastasis starts at the lesion size of about 2 mm. While X-ray mammography offers much better intrinsic spatial resolution, on the order of 50 micrometers, this anatomical imaging modality suffers from poor specificity to the type (cancerous or benign, etc.) of the suspicious structure seen on the mammogram. In some patients this X-ray imaging technique is not useful at all due to dense breast tissue, implants, or scars as the result of previously performed surgery. Breast MRI can provide more structural information than mammography but it also provides nonspecific information about the type of tissue imaged. Imaging techniques offering information as to the type or biology of the anatomical structures present in the breast and especially any suspicious lesions are based on biomarkers sensitive to molecular species present in the tissue. Examples of these functional or molecular imagers are Positron Emission Tomography (PET) scanners and gamma cameras. These standard imaging instruments are not capable of achieving spatial resolution in breast at the range of 2 mm or better, as is desirable, as mentioned above. It is therefore crucial to develop functional imaging techniques that can produce imaging power on the 1 mm resolution scale.
Although there has been proposed breast-specific functional imagers including dedicated compact gamma cameras placed close to breast, physical limits are imposed on their resolution by the parallel-hole mechanical collimator and the effect of distance between the imaged feature to the surface of the collimator. Radiotracer used in single gamma breast imaging with standard gamma cameras and with specialized breast-specific instruments is typically Tc-99m-Sestamibi, labeled with Tc99m, which emits gamma rays at 140 keV. Typically, the best average linear spatial resolution obtained in a compressed breast with these instruments using parallel-hole collimators is on the order of 5-6 mm. A partial remedy to this problem is imaging the compressed breast simultaneously from both sides, therefore reducing the average distance from any feature in the volume of the breast to the nearest collimator. However, even in this case the 2 mm limit is practically not obtainable due to interrelation between the collimator resolution and sensitivity. While parallel collimators offering 2 mm spatial resolution at a 2.5 cm distance can be made available, their sensitivity will be too low and the resulting images will exhibit too much statistical noise to allow efficient detection of small cancerous lesions. Other imaging schemes with pinhole collimators were proposed to attain better spatial resolution, but they still suffer from poor sensitivity due to the function of the pinhole gamma collimator.
FDA-approved dedicated breast PET imagers are available from Naviscan PET Systems of San Diego, Calif. These instruments are capable of achieving spatial resolution of <2 mm FWHM with F-18 positron-emitting radiolabels used for example in the F-18-fluorodeoxyglucose (FDG) biomarker. However, they suffer from another problem, related to the physics process of coincidence PET coincidence imaging, which results in poor sensitivity in the region of the breast close to chest wall. PET detection efficiency drops quickly off at the detector edges because of the geometrical requirement of simultaneous detection of two back to back 511 keV annihilation gamma rays produced in the act of positron annihilation with one of the electrons of the surrounding breast tissue. This produces a dead edge or sharp drop in detection efficiency when approaching the edge plane of the system. In comparison, single photon imaging does not suffer to the same extent from the chest wall effect and in addition one can use for example the slant collimator solution that will further reduce the dead region effect. This geometrical effect was defined in a recent clinical trial study as the cause of missing lesions placed at the base of the breast, close to the wall chest region. It is therefore a serious limitation and recognized deficiency of the dedicated PET breast imaging procedure as currently implemented with planar scanning modules.
Another example of a prior art breast imaging PET system is that described by RR Raylman, et al. in Development of a Dedicated Positron Emission Tomography System for the Detection and Biopsy of Breast Cancer, NIMA, 2006; 564(2):291-295. The imaging system includes two pairs of planar PET imager modules mounted on a rotating gantry under a patient's bed. The two pairs of modules in this proposed system have only limited reach towards the patient's chest wall and therefore, are unable to image the breast region closest to the chest wall.